It is known in the art that amphoteric latices can be prepared by aqueous emulsion polymerization. U.S. Pat. No. 3,404,114 discloses the preparation of amphoteric latices by (1) heating an aqueous system with polymerization catalyst and an emulsifier; (2) adding to the aqueous system an unsaturated carboxylic acid monomer and an unsaturated acid ester monomer; (3) neutralizing the mixture to a pH of about 7 with a neutralizing agent containing nitrogen; and (4) adding to the aqueous system an alkyl amino alkyl ester of an unsaturated acid monomer from the mixture.
Though this process produces stable latices, an appreciable percentage of undesirable grains accumulate during the polymerization or neutralization of the aqueous dispersion due to the instability of the polymer particles. These grains cause low filtration yields which lead to increased production costs, and the grains also decrease the quality of the dispersions.
The present invention addresses these difficulties by eliminating the intermediate neutralization stage, thereby producing stable amphoteric aqueous dispersions of synthetic polymers with less than 500 parts per million of undesirable grains, (about 0.05 percent of undesirable grains by weight of polymer). As used herein, the term undesirable grains refers to the total amount of grains which are retained after sifting the grains through a mesh screen of 40.mu. wide and a mesh screen 100.mu. wide.